Time-dependent aspects of nucleate boiling on a thin plate in well-wetted conditions have been studied for water at saturation, atmospheric pressure, and low heat flux by using simultaneous recordings of ebullition activity and wall temperature measured with a thermochromic liquid crystal. Existing heat transfer descriptions are not adequate in this case. Accounting for wall temperature variations leads to a new determination of the ebullition frequency and to a comprehensive but still non-predictive model of boiling heat transfer with two characteristic wall superheats, combining heat supply and activity of the nucleation site.